1. Field of the Invention
This invention relates to a constant-mesh transmission that uses electromagnetically operated latches and an associated computerized control to intelligently perform gear synchronization and gear shifting. This transmission can be used in a variety of automotive and industrial applications, and is mainly intended for automotive vehicles with internal combustion engines.
2. Description of the Prior Art
During a gear ratio change, prior-art automotive manual transmissions, either automated or not, use brute force to resynchronize the speed of the gear trains to the speed of the output shaft. In most prior-art transmissions, the synchronization is performed by friction cones placed both on the gears and the matching collars splined to the output shaft. This approach adds stress and wear to transmission components. An additional problem is that the transmission must be disengaged from the engine prior to the gear change, and then re-engaged after said change is completed. The power output of the engine must be reduced during said disengagement to avoid a speed spike, and then smoothly increased during the re-engagement to match the new speed of the gear trains. The duration and smoothness of the gear change very much depend on the skills of the driver or the sophistication of the automated shift system. In conclusion, the synchronization by friction cones affects the performance and drivability of the vehicle, and at the same time adds stress and wear to transmission components.
Another brute-force approach, mainly intended for racing applications, is the use of warts (dogs) on the collars (dog rings) and matching slots on the sides of the gears. Dogs and slots are forcibly engaged during gear shifts to bring the engine and transmission gear trains into synchronization with the output shaft. Although mechanically simpler and providing very fast gear shifts as compared to friction cones, the dog-type synchronization delivers harsh shifts and torque spikes, and causes high backlash in the transmission. Moreover, the dogs and slots are subject to high impact during engagement, which shortens their live and puts substantial stress on both the engine and the whole powertrain. These characteristics make dog-type transmissions unsuitable for mainstream automotive applications.
A recent design, disclosed in publication WO 2005/026570, seemingly reduces backlash and provides instant gear shifting when compared to earlier dog-type transmissions. Warts and collars are substituted by bars sliding in sleeves; slots in the gear sides are replaced by tabs that fit between the bar ends. Since the speed synchronization is made in a very short time, the impact between said bars and tabs during gear shifts is higher than in previous dog-type transmissions. Thus, harsh shifts, torque spikes, component wear, and stress on the engine and powertrain are even more severe than in earlier dog-type designs.
Publication WO 2005/005869 discloses a control system intended to mitigate the above mentioned torque spikes in the WO 2005/026570 transmission. This is a rather complex control system that monitors the torque transmitted through sensors in the transmission case and vehicle chassis, and mitigates torque spikes by slightly releasing the clutch during gear shifts. Obvious drawbacks of said control system are the additional mechanical and electronic parts required which increase the overall complexity and cost of this transmission.
Another important drawback in the WO 2005/026570 transmission is an inherent design limitation by which shifts can happen only between neighboring gears. This means that a transmission with N speeds requires (N−1) sleeves, and at least (N−2) shared gears (i.e. gears with tabs on both sides). If shared gears are not feasible, the number of gears is as high as (2N−2). In conventional manual transmissions shifts are also possible between non-neighboring gears, thus only N/2 or (N/2+0.5) collars are needed, and no shared gears are necessary. For example, a five-speed WO 2005/026570 transmission would require four sets of sleeves with bars (1st to 2nd, 2nd to 3rd, 3rd to 4th, and 4th to 5th), and three shared gears (2nd, 3rd and 4th). If shared gears are not feasible, as much as eight gears would be necessary instead of the usual five. By contrast, a conventional five-speed manual transmission needs only three collars (1st to 2nd, 3rd to 4th, and 5th), the latter usually shared with the reverse gear. The increased number of sleeves in the WO 2005/026570 transmission requires a longer output shaft, and thus a bigger and heavier transmission case; the effect of any additional gears on the transmission size, weight and complexity is even more substantial, since these are actually pairs of meshed gears.
The present invention discloses a compact latch mechanism, an electromagnetic actuator to operate said latch mechanism, and an associated computerized transmission control unit (TCU) to command said actuator and, more importantly, to synchronize the speed of the engine and gear trains with the speed of the transmission output shaft. The TCU cooperates with the engine control unit (ECU) in such a way that speed differences between—and torque transmitted through—the gear trains involved and the output shaft are minimal during gear ratio changes; this permits effortless and on-the-fly coupling and decoupling of gears to/from the output shaft, without requiring any clutch operation. The shafts and gears are arranged in the same way as in conventional constant-mesh transmissions.
The proposed latch-gear set is more compact than the conventional collar-gear set with friction cones. Similarly, the proposed electromagnetic actuator takes less space than conventional forks and actuators, either manual or automated. Thus, this transmission is smaller than an equivalent prior-art transmission with the same number of speeds. Or, the other way around, this transmission can pack more gears into the same space, which means more speeds in a given transmission case.
The transmission disclosed herein is in fact an automated manual transmission that overcomes known drawbacks of prior-art automated implementations: sluggish shift feel, and bulky and complex actuators for gear shifting and clutch operation.
Different approaches have been proposed to improve the slow shift in automated manual transmissions. The recent U.S. Patent Application 20040106498 (Badillo et al.) describes a method to reduce speed flare during the engine torque reduction phase prior to a gear shift. This method seemingly reduces shift delays coming from the engine side, but obviously cannot eliminate the time needed to disengage and then re-engaging the clutch, which is inherent to the already described friction-type synchronization. By contrast, the transmission disclosed in the present invention does not require any clutch operation during gear shifts, and thus does not incur in clutch-related delays.
U.S. Pat. No. 6,886,424 (Janson et al.) proposes a second layshaft with gears and a one-way clutch to transmit torque during gear shifts. This approach avoids torque interruptions during said shifts at the expense of substantial added mechanical complexity, size and weight. The transmission provided in the present invention offers comparable shift quality in a much simpler and compact mechanical arrangement.